HONG KONG(Reuters) - The corruption crackdown ordered by Chinese President Xi Jinping is reliant on a secret system of detentions and torture beyond the reach of the formal Chinese criminal justice system, a U.S.-based human rights group said in a report on Tuesday.

Human Rights Watch, which released its study in Hong Kong, called for the abolition of the system known as shuanggui, through which it alleges that confessions from Communist Party members are coerced. It said at least 11 people, according to media reports, had died under shuanggui since 2010.

"President Xi has built his anti-corruption campaign on an abusive and illegal detention system," Sophie Richardson, China director of Human Rights Watch, said in a statement.

"Torturing suspects to confess won't bring an end to corruption, but will end any confidence in China's judicial system."

Human Rights Watch says the 102-page report is the first on the secretive shuanggui system to be based on interviews with former detainees and their family.

"They kept telling me to 'explain problems'," says one detainee quoted in the report. "How much money did I receive. They made me make it up. I had to make it up - if I didn't, they'd beat me."

China's graft-busting Central Commission for Discipline Inspection and the Foreign Ministry did not immediately respond to requests for comment on the report.

The Chinese government has acknowledged there is a problem with torture in its legal system and has tried repeatedly to crack down on it, most recently in October.

President Xi Jinping came to power four years ago vowing to launch a campaign to root out deeply ingrained corruption, warning that the problem had grown so bad it threatened the ruling Communist Party's grip on power.

The crackdown has targeted a broad range of high ranking officials, from various ministry chiefs to military brass and former judges, as well as numerous bosses of state-owned firms.

The Human Rights Watch report details disappearances and extensive use of torture, including deprivation of sleep, water and food, as well as beatings.

"They didn't let me sleep. I had a total of 10 days without closing my eyes," said one former detainee interviewed for the report.

The Human Rights Watch report describes detainees being forced into tiny rooms with no natural light or being made to sit in stress positions for extensive periods, leaving them with swollen legs and festering wounds.

The report included 21 interviews with four former detainees, analyzed 38 court verdicts and 35 detailed detainee accounts from 200 Chinese media reports.

China's shuanggui, literally "double set", is an internal party investigative technique under which the use of force is technically forbidden. But it is condoned in party disciplinary regulations as being necessary when evidence is scant or there is a risk of collusion, evidence tampering or flight.

The report said that shuanggui "not only facilitates serious human rights abuses, it depends on them". The threat of being subjected to the system "strikes fear in the Party members regardless of their position," it said.

The practice remains controversial within China's legal community where many see it as unconstitutional. Some argue it is a tool to carry out inter-factional purges within the party.